A vertical cavity surface-emitting laser device emits light in a direction perpendicular to a substrate, which has attracted attention in recent years because the vertical cavity surface-emitting laser device is less expensive, lower in power consumption, smaller in size and more suitable for a two-dimensional device than an edge emitting type semiconductor laser device that emits light in a direction parallel to its substrate.
Application fields of the surface-emitting laser device may include a light source for an optical writing system in a printer (which has an oscillation wavelength in the 780 nm band), a light source for writing in a light disk apparatus (which has an oscillation wavelength in the 1.3 μm band or in the 850 nm), and a light source for an optical transmission system such as a LAN (i.e., Local Area Network) using an optical fiber (which has an oscillation wavelength in the 1.3 μm or in the 1.5 μm band). Furthermore, the surface-emitting laser device is expected to be applied to a light source for optical transmission between boards, within a board, between chips in a LSI (i.e., Large Scale Integrated circuit) and within a chip in a LSI.
In these application fields, light emitted from a surface-emitting laser device (which may be called “emitted light” hereinafter) is often needed (1) to have a round cross-sectional shape, and (2) to have a constant polarization direction.
To make the cross-sectional shape of the emitted light round, controlling a high-order transverse mode oscillation is necessary. To do this, various approaches have been tried as disclosed in Japanese Patent Publication No. 3566902 (which is hereinafter called Patent Document 1).
Also, various approaches to control the polarization direction of the emitted light have been tried as disclosed in Japanese Patent Publication No. 3955925 (which is hereinafter called Patent Document 2).
Furthermore, balancing the control of the high-order transverse mode oscillation and the control of the polarization direction has been studied as disclosed in Japanese Patent Application Publication No. 2007-201398 (which is hereinafter called Patent Document 3) and Japanese Patent Application Publication No. 2004-289033 (which is hereinafter called Patent Document 4).
However, the methods disclosed in Patent Document 1 and Patent Document 2 have difficulties in balancing the control of the high-order transverse mode oscillation and the control of the polarization direction. Moreover, the method disclosed in Patent Document 3 raises the fear that electric resistance of the surface-emitting laser device increases or that operating life duration decreases because of an increase of current density. Furthermore, the method disclosed in Document 4 has difficulty in stabilizing various characteristics of the surface-emitting laser device or a control characteristic of the high-order transverse mode.